In this proposal, we describe experiments which attempt to discover how the photoreceptor generates its response. We shall be making intracellular and extracellular recordings from Limulus and Bufo marinus, in order to characterize the conductances in the receptor membrane. We shall attempt to determine the ion specificity of the light-dependent conductance rods. We shall explore the function of light-dependent enzymatic reactions in the isolated retina. We shall investigate the response properties of mammalian receptors by attempting to record both intracellularly and extracellularly from rods in the isolated retina. We propose experiments which attempt to identify the receptor synaptic transmitter. Finally, we shall be investigating the pharmacology of photoreceptors by studying the effects of drugs and toxic agents on their responses.